Composite video signals include a synchronization component or a sync signal. The sync signal must be separated or sliced from the composite video signal, usually at a fifty percent level which is preferably halfway between the blanking level and the sync tip level. This is usually implemented by systems which clamp the sync tip to a known voltage and then sample the blanking level. Unfortunately, the clamps commonly used for this purpose have a number of disadvantages, e.g. the clamped output voltage tends to droop, and the droop varies with the input duty cycle. In addition the clamps commonly used cause sync tip distortion, and the clamped output tends to ride on the sync tip noise.
In addition, precision sync separator circuits are required to accurately slice or separate the input video signal sync pulse at the fifty percent level for signal amplitudes ranging from 0.5 volts peak to peak to 2.0 volts peak to peak or higher (e.g. 4 volts p--p). When the signal amplitude changes rapidly, e.g. from 2.0 Vp--p to 0.5 Vp--p, the sync separator slicing level can be higher than the sync amplitude in a typical slow recovering sync separator circuit. This can result in incorrect slicing of the video signal and loss of the sync signal.
In addition, difficulty can be encountered in reliably detecting the presence of a video signal. In the design of a video detector circuit, appropriate criteria must be chosen to determine whether a valid video signal has been received. If the evaluation criteria are too strict, a valid video signal corrupted with impulse noise and random time base errors will not be detected. However if the evaluation criteria are too loose, large amplitudes of pure noise may be detected as a valid signal.